Carbonate Sedimentology Lecture 15 - PDF

Carbonate Sedimentology EA2110 / EA3110 LECTURE 15: 16 TH SEPTEMBER 2024 Carbonate Sediments and Sedimentary Rocks Carbonate sediments are precipitated from water through chemical or biogenic processes Carbonate minerals are those who have a metallic cation bonded with a carbonate (CO32-) anion...

Carbonate Sedimentology EA2110 / EA3110 LECTURE 15: 16 TH SEPTEMBER 2024 Carbonate Sediments and Sedimentary Rocks Carbonate sediments are precipitated from water through chemical or biogenic processes Carbonate minerals are those who have a metallic cation bonded with a carbonate (CO32-) anion → Calcite and its polymorphs (e.g., aragonite) are common Adapted from illustration by Dr. Bruce Railsback 2 Carbonate Mineralogy Ca+2 + 2HCO-3 CaCO3 + H2CO3 calcium ion + bicarbonate ion calcite + carbonic acid These ions are a biproduct of chemical weathering Carbonic acid is also made when water and carbon dioxide mix → Problem: pushes reaction back towards dissolution Fossiliferous limestone (gastropods and bivalves) Credit: P. Getty, Collin College 3  Carbonate Mineralogy Calcite vs Aragonite → Calcite has rhombohedral cleavage (3 directions) → Parallel extinction in thin section → Aragonite has only 1 easily observed cleavage direction Soldati et al. (2016) 4  Carbonate Sediments and Sedimentary Rocks Limestone is the most common carbonate rock → Made in lots of ways, both biogenic and chemical → Non-marine variants include travertine and tufa (both are chemically precipitated) → Marine variants are dominantly biogenic but can be chemical sediments too → Composted of material made from carbonate minerals (calcite, aragonite, etc.) Lakes Caves Ooidic limestone (ooids and crinoid) Credit: M. Wilson, College of Wooster 5  Carbonate Sediments and Sedimentary Rocks Credit: Great Barrier Reef Marine Park Authority 6 Carbonate Classification: Terminology Allochems = carbonate sand Usually consists either: → Skeletal remains / fragmented skeletal remains (broken bits of shell) → Non-skeletal grains Micrite = carbonate mud → Commonly the product either of chemical precipitation or algal/bacterial activity → Can be very finely broken-down bits of shell (sub-microscopically) Sparite = carbonate cement → Formed either shortly after or longer after deposition 7 Allochem Environments 8 ALLOCHEMS, MICRITE, AND SPARITE Skeletal Allochems Skeletal allochems are carbonate fragments made from biogenic processes (generally shells, corals, and other skeletal elements) Molluscs (bivalves, gastropods, cephalopods) Crinoids (sea lilies; starfish-like animal) Plankton (forams, coccospheres, diatoms) Coral (branching, encrusting, etc) Stromatolites and calcareous algae 10 Skeletal Allochems: Bivalves Bivalves are a type of filter-feeding mollusc that characteristically grow two shells connected by a hinge Stephen Frink/Photodisc/Getty Images 11 Skeletal Allochems: Gastropods Gastropods are a group of molluscs represented by snails some of which grow spiral shells that protect the organism Barrett & MacKay / All Canada Photos / Getty Images 12 Skeletal Allochems: Cephalopods Cephalopods are a group of molluscs that have tentacles →Includes octopus, cuttlefish, nautilus, ammonites, and belemnites 13 Skeletal Allochems: Echinoderms Echinoderms are a type of marine invertebrate with hard often spiny exterior (includes sea urchins, starfish, sand dollars, etc) 14 Skeletal Allochems: Crinoids Crinoids are a type of echinoderm that attaches to the seafloor →Stalked varieties are commonly called sea lilies 15 Skeletal Allochems: Plankton Plankton are tiny organisms carried by tides and currents Foraminifera (forams) make silicious or calcareous tests (shells) Coccospheres are the most productive calcifying organism Diatoms are unicellular algae that can form small colonies Radiolaria (a type of plankton with silica tests) Coccolithophore (plankton) 16 Skeletal Allochems: Plankton 17 Skeletal Allochems: Coral … 18 Non-Skeletal Allochems Non-skeletal allochems are carbonates made from chemical precipitation 19 Non-Skeletal Allochems: Ooids Chemically precipitated in super-saturated shoaling environments 20 Non-Skeletal Allochems: Ooids Ooids are made when ocean water becomes supersaturated in carbonate ions and precipitation is triggered by shoaling swash action → Ooids are concentrically banded grains of chemically precipitated carbonate → Can think of ooids as tiny grains that are kinda like Jawbreaker candy but made of carbonate minerals instead of sugar 2mm 21 Non-Skeletal Allochems: Ooids An excellent example of where ooids form is in the vibrant blue (carbonate saturated) shoaling water of the Bahamas 22 Non-Skeletal Allochems: Pisoids and Oncoids Pisoids are the same as ooids but larger than 2 mm in diameter (usually slightly less spherical) Oncoids are similar to pisoids but formed by algal growth (less concentric than ooids or pisoids) 23 Non-Skeletal Allochems: Peloids Peloids are internally structureless, generally spherical aggregates of micrite (e.g., fecal pellets) 24  Non-Skeletal Allochems: Intraclasts Intraclasts are partially lithified carbonate sediment derived from within the basin (e.g., during storms) Micron-scale intraclasts Deepwater intraclasts 25 Micrite: Carbonate Matrix Micrite as matrix between fossil Micrite is microcrystalline calcite (also called lime mud) fragments → Consists of carbonate particles < 4 microns (i.e., clay sized) 26 Sparite: Carbonate Cement Provides excellent information about rock history. More details in next lecture! 27 CARBONATE CLASSIFICATION Carbonate Classification 1. Folk (1962) Classification: Independent of formation processes 2. Dunham (1962) Classification: Concerns depositional environments using micrite % 3. Embry-Klovan (1971): Expansion on depositional textures (combination!) 4. Combined Embry-Klovan and Dunham Classification Carbonates can also sometimes be classified in a similar way to siliciclastic sediments when referring to transported, abraded carbonate grains. This approach uses grain size as follows: Calcilutite: grains < 63 μm Calcarenite: grains >63 μm but < 2mm vs Calcirudite: grains > 2 mm This approach will NOT be used in this class 29 Biogenic Limestone: Coquina Coquinas consists of sand and/or gravel sized particles of shell fragments and other carbonate material → Specifically does NOT have siliciclastic sand in it → Common form of beach-rock (lithified in situ) beach-rock Coquina beach-rock formed in situ on Esk Island, QLD 30 Folk (1962) Classification Emphasizes what carbonate looks like in thin section → Does not incorporate formational processes → Based on observable textures with minimal interpretation Folk also provided extra textural descriptors: 31 Dunham (1962) Classification Less concerned about the particles, more particular about the proportion of mud → Reflects environmental conditions during deposition → Originally didn’t delineate within boundstone 32 Embry-Klovan (1971) Modified the Dunham Classification to include descriptions of Dunham’s boundstones Allochthonous: material broken apart into pieces but are still larger than 2 mm Autochthonous: formed in situ 33 Combined Embry-Klovan and Dunham Classification Schemes 34 Nichols, 2009 35 Next Lecture: Sedimentary Diagenesis